1. Field of the Art
This invention relates to a gas blowing tuyere to be used in bottom or side walls of various metal refining furnaces or molten metal containers such as ladles and the like.
2. Description of the Prior Art
There are many types of containers for holding molten metal for refining, lagging, storing, transporting or for other purposes. For example, in addition to LD converters, there are known a diversity of converters, including LF furnace, VAD furnace, AOD furnace, ASEA-SKF furnace and RH and DH vacuum melters. Among known molten metal containers other than those refining furnaces are ladles, metal mixers, mixer cars and the like. Most of such molten metal containers more or less require stirring the content constantly or intermittently. Of the various mechanical and gas stirring systems which are employed in the art, the present inventors conducted an extensive study on the gas stirring particularly in refining processes in top and bottom blown LD-converters using oxygen for top blowing and an inert gas or oxygen wrapped in a cooling gas for bottom blowing, and as a result arrived at some conclusions. More specifically, a study on the tuyere construction suitable for bottom blowing an inert gas or oxygen in the LD-converter has succeeded in determining a tuyere construction which permits to set or vary the blowing gas flow rate over a wide range and to suppress the erosion of the tuyere itself and the surrounding refractory material to a significant degree. Further, experiments on the tuyere construction according to the present invention have revealed that it is widely useful for various molten metal containers other than LD-converters.
The converters which are designed to blow pure oxygen into molten metal are generally classified as a top blowing type and a bottom blowing type, of which the top blowing type has been more popular in the art although both have long histories of use. However, the bottom blowing type converter are increasingly accepted these days to utilize the stirring effect peculiar to the climbing streams of the bottom blown gas. Namely, it has been revealed that metallurgical reactions are improved to a significant degree as a result of the positive stirring actions of the climbing gas streams on the molten steel and slag, as compared with a pure oxygen top blowing type converter. Therefore, there is even a trend of entirely replacing the top blowing type converters by the bottom blowing type. The present inventors have conducted a study of the top and bottom blowing converters in an attempt to develop a new refining process which incorporates top and bottom blown gases parallelly in such a manner as to secure the advantages of the bottom blowing while retaining the merits of the top blowing, for instance, versatility in refining.
In advancing research on the top and bottom blowing converters, either one of the following two approaches which are conceivable in this connection was adopted in consideration of the conditions of an iron works.
(1) A system in which several to several tens percent of the total quantity of feed oxygen is blown in through the bottom; or
(2) A system in which the total quantity of feed oxygen is used for top blowing while blowing in an inert gas through the bottom at a relatively low flow rate (e.g., at a rate of 0.01-0.2 Nm.sup.3 /min per ton of a charge).
The enhancement of the stirring action by the bottom blowing brought about the following effects.
(A) The composition and temperature of the molten bath were maintained uniform in the entire areas of the furnace, improving the success rate of attaining a target composition at turn down.
(B) The efficiency of oxygen which is consumed in decarburization reactions was enhanced, lowering the prime consumption of the refining oxygen.
(C) The percentages of the T.Fe component in the slag at turn down was reduced, improving the yield of steel.
(D) The O-content of steel was reduced and the Mn-content was increased at turn down. Therefore, it became possible to reduce the amounts of Al and Fe-Mn which were added for adjusting the composition.
(E) The dephosphorization capacity of the slag was improved, permitting a reduction in the prime consumption of a subsidiary material like calcined lime.
Although the above-mentioned effect of improving metallurgical reactions was largely influenced by the flow rate of the bottom blown gas, it was only produced in a conspicuous degree up to a flow rate of approximately 0.05 Nm.sup.3 /min per ton of molten steel in the case of an inert gas bottom-blowing system, with no remarkable improvement in that effect occurring even if the flow rate of the bottom blowing gas were increased further. Rather, in the case of a high carbon steel with a turn down C-content greater than 0.60%, the T.Fe content of the slag was reduced considerably at turn down, giving rise to a problem concerning degraded dephosphorization capacity. Therefore, studies were carried out in search of the bottom blowing conditions free of the adverse effects on the dephosphorization capacity, repeating extensive experiments. As a result, it was found that the advantages of the bottom blowing can be acquired without causing the above-mentioned problems by restricting the flow rate of the bottom blowing gas to about 0.1 Nm.sup.3 /min versus one ton of molten steel when refining a high carbon steel.
With regard to the tuyere construction for the bottom blowing, there are known in the art (I) a tuyere consisting of a single tube and (II) a tuyere which includes concentric double tubes. The former is used for exclusively blowing in an inert gas, while the latter is used for blowing in oxygen through the inner tube and a protecting or cooling gas through the outer tube. These tuyeres, however, have the following drawbacks when used for blowing an inert gas. Referring to FIG. 1 which illustrates a mono-tube tuyere 1 as embedded in a refractory bottom wall 2 of a furnace, the molten steel 5 in the vicinity of the bottom wall 2 is partly solidified by the primary cooling action of the blown-in gas, forming a mushroom (a mass of base metal) as indicated by 3. The blown-in gas is injected into the molten steel 5 through narrow gas passages 4 which are formed in the mushroom 3, and climbs up through the molten steel 5 in the form of bubbles 6. In some cases, however, the gas passages 4 are not sufficiently formed due to an increased resistance of the mushroom 3 and the blowing is blocked with relatively high frequency, failing to blow in the gas in a stable condition. In order to avoid this problem, the back pressure of the tuyere has to be raised to a level higher than 10 kg/cm.sup.2 G in the case of a mono-tube tuyere, although it depends on the static pressure of the molten steel. On the other hand, if the flow rate of the blowing gas is to be maintained at a value lower than 0.1 Nm.sup.3 /min versus one ton of molten steel as mentioned hereinbefore, it becomes necessary to make the tuyere hole diameter smaller. In order to satisfy these requirements in a process which involves control of the blowing gas flow rate over a wide range, there have been imposed further restrictions, i.e., a pressure increase to a range over 10 kg/cm.sup.2 G for stable blowing operation and the use of a blowing facilities which are calibrated to an extremely high pressure.
In an attempt to solve these problems, extensive experiments have been conducted using a tuyere of concentric double tubes (FIG. 2) instead of the above-mentioned mono-tube tuyere, and it was found that an aimed gas flow rate can be secured in a relatively stable manner by maintaining the back pressure of the outer tube at a predetermined high level, without extremely reducing the opening diameter of the inner tube.
The double-tube tuyere has been effective particularly for simultaneously blowing in a gas and powder or the like, in addition to injection of a large quantity of oxygen or the like. However, even the concentric double-tube tuyere has a problem in that the gas flows from the inner tube have a large influence and in some case the blowing operation is thereby rendered unstable. Therefore, it is unsuitable particularly for blowing in a gas at a relatively small flow rate or for controlling the gas flow rate over a wide range, as manifested, for example, by the results of experiments shown in FIGS. 3 and 4.
More specifically, the graphs of FIGS. 3 and 4 show variations in the gas flow rates of a concentric double-tube tuyere which is anchored in a bottom wall of a converter to blow in oxygen through the inner tube and a cooling CnHm gas through the outer tube, detecting the blowing gas pressure in pipings in the vicinity of the tuyere. Although no large variations in flow rates are observed in FIG. 3, the blowing becomes unstable with extremely large variations in the inner and outer tube pressures Ip and Op in the case of FIG. 4 where the flow rate of oxygen through the inner tube is about 1/2.5. It will be understood therefrom that the use of a concentric double-tube tuyere does not give sufficient solutions to the above-mentioned problems. Besides, the convention tuyeres have a detrimental drawback in that the refractory wall around the tuyere is worn out considerably by the actions of the gas jets which are injected into the molten steel, particularly by the bottom beating actions (back attacks) of the downward streams which are formed immediately after injection. In view of these circumstances, it has been concluded that the conventional tuyeres are defective in construction for blowing in an inert gas or oxygen, and a tuyere of the afore-mentioned novel construction has been provided as a result of extensive studies and experiments.
With regard to the literatures disclosing converter tuyeres, Japanese Laid-Open Utility Model Specification No. 55-142554, and 54-110608, Japanese Laid-Open Patent Specification No. 50-87908 and Japanese Patent Publication Specification Nos. 43-29843 and 49-21002 are cited here as references of interest.